Optical disk devices are used for the storage of computer-prepared data and have recognized value in their ability to store large quantities of data. The media for use in such devices is reactive to bursts of light, such as may be produced by the rapid switching of a semiconductor laser. In order to write data on optical media, the laser power must be controlled at a fairly high power level, in order that the media can be altered so that it reflects light indicating the presence or absence of data. In reading the data back, the laser power level is controlled to a lower level so that the media is not altered by the laser beam.
Optical media is of two general types, media which can be written only once and media which can be written, erased, and written again. Write-once media (WORM) is permanently altered when write power levels are produced by the laser beam. Erasable media, such as magneto-optic (MO) media, is not permanently altered when data is written. In the MO media, the magnetic orientation of the reactive material is altered in the writing process, and in the erasing process, the magnetic orientation is reordered.
In operating an optical disk system, it is necessary to set the correct laser power level to read and to write for each optical disk. The correct parameters for the optical disk are included in information in an identification header stamped onto the disk, itself. That information, when read by the system, enables a calibration circuit to set the desired current levels for the laser to produce correct laser power. Since, however, the laser is subject to unintended changes in its operating parameters, particularly with temperature and aging, the calibration method is also used to change current levels for the laser so that the correct power level is maintained under operating conditions and throughout laser life.
The common practice of calibrating laser circuits to operate with a given optical medium usually involves analyzing the laser light intensity at the optical medium. To do that, laser control circuits are set to match a predetermined or desired light intensity at the optical medium. Analysis is conducted to enable setting digital-to-analog converters (DACs) which control laser power in the writing and the erasing operations of optical mediums.
Read power levels and the power level to write zero digits, called a baseline level, are also established. However, as the efficiency of the laser changes, the read power produced for a given laser current changes. Changes in efficiency result from the laser moving in and out of focus, a situation causing feedback noise within the system. Noise is created from small variations in the optical path length which occur as an optical disk rotates and also by differing reflections from the media. The semiconductor laser chip, itself, may be inherently unstable in that emission from the laser may be reflected back into a secondary cavity with the consequence that read power emitted by the laser may fluctuate.
In order to eliminate the effects of feedback noise, it is common practice to modulate the laser, that is to turn the laser off as light is returned to the laser cavity from the optical disk medium. The instant invention relates, in part, to automatically calibrating the amplitude of the modulation, that is modulation depth, so that a correct modulation depth is maintained in the system without manual intervention. Thus, a technique is presented in which the laser driver, itself, may calibrate the modulation depth as temperature changes, as the read power level changes, and as laser characteristics change, for example, with age.
A provision is also made for compensation to eliminate switching transients as laser power is switched from one power level to another. For example, when the power level is switched from a read power level for MO media to a baseline write power level, laser current and power may fluctuate during a transient period. This invention provides a circuit which effectively eliminates such transients.